1. Field of the Invention
This invention relates to improvements to radiant energy collecting apparatus and in particular to an improved solar energy concentrating apparatus.
2. Background Art
Solar collectors are often built in a manner to achieve high temperatures in the energy collecting medium. Concentration of solar energy has been achieved in the past using parabolic concentrating mirrors. Cylindrical parabolic concentrators are commonly used as they are inexpensive, however, such concentrators cannot achieve high concentration ratios. High concentration ratios can, however, be easily achieved by the use of spherical, parabolic concentrating mirrors, however such mirrors are excessively expensive for solar heating applications.
To achieve high concentration ratios with a cylindrical parabolic mirror system having primary cylindrical parabolic concentrators, an additional secondary series of cylindrical parabolic mirrors can be placed at right angles to the focus line of the primary mirrors and placed at the point of focus of the primary mirrors. The secondary mirrors concentrate the energy on collecting pipes carrying a heat absorption medium. Such an arrangement is shown in U.S. Pat. No. 4,281,640 to WELLS.
With the sun at any position, other than directly at right angles to the focus line of the primary mirrors, that is directly overhead, however, the secondary parabolic mirrors cannot be placed so that more than a small proportion of the reflecting surface of the secondary mirrors will be at the focus line distance from the primary mirrors.
It is thus necessary to extend the length of the individual secondary mirrors to trap the wider area of sunlight from the primary mirrors as large areas of the secondary mirrors will be out of focus with the light from the primary mirrors. Additionally the length of the collecting pipe associated with the secondary mirrors is required to be increased.
Maximum concentration ratios available from the primary mirrors cannot be utilized by the secondary mirrors and this significantly reduces the total maximum concentration ratio from this configuration. In addition the longer length of the individual secondary mirrors substantially increases their cost. Furthermore, increased heat loss occurs from the longer length collecting pipes.
In the construction of solar energy concentrators the heat collecting and conveying medium is usually conveyed through a system of pipes. The heat collecting section of pipe on which the solar energy is concentrated is by necessity at high temperatures and is particularly prone to re-radiated heat losses. To minimize such heat losses the heat collecting pipe is often enclosed within a glass tube and the space between the inner collecting pipe and the glass tube is maintained at a high vacuum. The maintenance of this vacuum to minimize heat losses is generally essential and often difficult to achieve and maintain. The cooler outer glass tubing and the high temperature of the heat collecting pipe create uneven longitudinal heat expansion variations rendering the outer tubing vulnerable to shattering.
It is an object of the present invention to overcome or substantially ameliorate one or more of the above-described disadvantages and to enhance the usefulness of such systems in the field.